A person may walk through a geometric physical area—such as a retailer location, store, shopping mall, grocery store, etc.—and interact (e.g., browse, carry, try on, sample, etc.) with articles (e.g., clothing, home goods, sporting goods, electronics, books, etc.) in the marketplace. In some examples, sensor systems installed at the geometric physical area may be used to track the person. Such tracking may include measurements (e.g., images from cameras) to detect when the person enters or leaves the geometric physical area.